High power light emitting devices, containing light emitting diodes (LEDs), are used in an increasing number of illumination applications today. Generally, two material systems for making high-power LEDs are used.
InGaN is used to produce efficient blue LEDs
AlInGaP is used to produce efficient red and amber LEDs.
Both material systems suffer from severe losses in efficiency when their material composition is changed to shift the emission wavelength from blue towards green and from red towards green.
By application of wavelength converting materials, such as fluorescent and/or luminescent materials, in the path of light, the emitted wavelength can be adapted to many specific wavelengths. Blue and/or UV-light emitting LEDs are especially suitable as the light source in such light emitting diodes (or wavelength converted light emitting diodes) due to that the wavelength converting materials typically absorb at least part of the light emitted by the diode and emits light having a higher wavelength (a red-shift).
The InGaN systems can be combined with such a wavelength converting material, or optical component, for example a phosphor material, to convert part of the high energy, low wavelength blue light to lower energy, higher wavelengths. In such a way, by combining the blue LED with appropriate phosphor bodies on the LED, white LEDs can be produced (typically using YAG:Ce phosphors), or a blue LED can be converted to a green, yellow, amber or red LED using suitable phosphor materials. This color conversion comes with efficiency losses (mainly Stokes shift loss), but the high starting efficiency of blue LEDs makes even full-conversion to amber and red an attractive alternative to direct emitting AlInGaP systems that suffer from thermal efficiency issues.
JP 2002353507 discloses a light emitting body, wherein a fluorescent substance, which changes the light emitted to another color, is stabilized. This is achieved by filling slots inside a LED by a resin containing phosphor, as a die bonding agent to stabilize the total amount of the resin.
Conventional LED phosphor technology uses phosphor pigments or powder particles embedded in a resin on top of the LED. This leads however to backscattering losses and processing variations. A new technology makes use of a ceramic phosphor technology called “Lumiramic” technology (a Lumiramic converter is described in US2005/0269582 A1). This technology makes it possible to produce highly photo—and thermally stable ceramic phosphor platelets with well defined thickness and geometry to match the LED geometry, which is typically square, by 1×1 mm. By controlling the porosity in these ceramic phosphor bodies, or phosphors, the path length differences with angle can be sufficiently scrambled/scattered to provide a rather uniform over-angle color performance while sacrificing some light through backscattering towards the LED.
By using the Lumiramic technology, white LEDs can be made (using e.g. YAG:Ce phosphors) by partial conversion of blue light to higher wavelengths. Also, green, amber and red LEDs can be made by attempting to fully absorb the blue LED light and efficiently convert it to a color spectrum matching green, amber or red characteristics.
However, this platelet phosphor technology requires a non-negligible thickness of the phosphor body compared to the size of the LED. The phosphor body typically has a thickness in the order of 120 μm in a size of 1×1 mm for a white LED. This results in a significant contribution of the light emission from the four lateral, or side, surfaces of this squared volume.
Furthermore, the LED itself has a side surface with non-negligible light extraction. The LED chip may be of the “flip-chip” type, where both leads are positioned on the same side of the chip. This design facilitates the arrangement of the wavelength converting body on the light emitting surface of the device. In “flip-chip” LED-technology, the LED is mounted with the substrate, or the light transmissive body, thereon. When this substrate, typically sapphire, is not removed, this sapphire substrate of a thickness of typically 100 μm also gives a significant side surface contribution. To solve this issue, the substrate may be removed in a lift off process. Still, the InGaN LED stack, consisting of quantum wells and anode, cathode and reflector, may have a thickness in the order of 10 μm and consist of materials with high refractive index, resulting in considerable waveguiding and non-negligible side emission.
A bonding layer, connecting the LED and the Lumiramic phosphor, adds to the side surface thickness and typically has a thickness of 10 μm.
Examples of bonding materials include for example a silicone resin.
The disadvantages relating to the light emitted from the lateral (edge) surfaces of a light emitting device are as follows:
Non-converted light, such as blue light, leakage from the edge surfaces arising from the LED edges and the bond edges. For partial conversion Lumiramic, this may result in excess of blue light as well as significant variations of blue light flux present at large angles with respect to the normal direction and therefore reducing the over-angle color uniformity and consistency. Especially layer thickness variations, such as bond thickness, and phosphor placement inaccuracies as occur in processing yield variations of blue light leakage from the side faces. For full conversion Lumiramic, blue light leakage highly reduces the color purity of the green, amber or red LED. In addition, this light leakage reduces the efficiency as part of the blue light is not converted to the desired color.
A wavelength conversion through the lateral edges of the phosphor that is different in spectrum compared to the conversion spectrum from the top surface of the phosphor, due to path length differences between light emitted from the sides and from the top surface. This is especially unwanted for full conversion phosphors, as incomplete conversion through the phosphor sides reduces color purity of the LED.
A light flux emission from the side surfaces that is partly (roughly by half) directed downward, back to the submount that is usually located next to the LED die. Generally, such light emitted towards the wrong side, as well as light emitted to the top direction but at large angles with the normal direction is hard to capture effectively in an optical system combined with the light source, such as collimator optics, lenses etc., and therefore is likely to reduce system efficiency. Similarly, the downward light flux interacts with the submount and typically will be partly absorbed, partly reflected and usually affected in color by the interaction with the submount surface. The light scattered or reflected from the submount also increases the LED source area and results in stray light, which is undesired for etendue critical applications, such as automotive front lighting or projection LED systems.
An increased etendue compared to the active LED surface area. This is caused by the increased surface area of the phosphor surface compared to the surface area of the LED. The phosphor sides will contribute to an increased source area even if the phosphor top surface area is similar to the LED. This is especially important in etendue critical applications, such as automotive front lighting, camera or video flash modules or projection LED systems.
In conclusion, the various embodiments of light emitting devices all suffer from disadvantages related to the side edges of the Lumiramic and/or the bonding layer and/or LED die. These disadvantage are mainly related to color variations or limited color purity due to unwanted spectral differences between the side-emission and the top-emission. Moreover, there will be a wavelength conversion light flux emission from the side surfaces partly (roughly by half) directed downward and sideward that is hard to use effectively in an application. In addition, the etendue may also be increased compared to the active LED surfaces area, which is a disadvantage in etendue critical applications, such as projection LED systems, automotive headlamps or spot lights.
A method for coating a light emitting device has been disclosed in US 2005/0062140, using a mold for applying materials with light conversion particles on an LED device. However, this method involves a specific coating apparatus, and is laborious and expensive.
Thus, there is a need for a light emitting device and methods for producing such light emitting devices that does not suffer from unwanted color variations or purity and efficiency losses due to light emission through the lateral edges of the light emitting device, or gives increased etendue compared to the active LED surface area.